1. Field of the Invention
This invention is directed to a coating composition, in particular, to a coating composition useful as a primer, a primer surfacer or a primer filler having excellent chip resistance and good sandability. Also, this invention is directed to a clear coating composition useful as an exterior top coat applied over a pigmented base coat that has excellent appearance, outdoor weatherability, scuff and mar resistance, and in particular water spot resistance that develops into a hard tack free finish in a relatively short period of time after application to a substrate making the composition particularly useful in the refinishing or repainting of automotive or truck bodies or parts thereof.
2. Description of the Prior Art
To meet the many requirements of an exterior finish for automobiles, the automotive industry currently is using multi-layer finishes on automobile and truck bodies and parts. Typical of these finishes are layers of the following: (1) an electrocoat layer applied over a substrate, typically, a phosphatized cold rolled steel, (2) a primer layer, (3) a colored layer, typically pigmented, and (4) a clear layer. A colored top-coat layer may be used in place of the colored layer and clear layer. On repairing or refinishing such multi-layer finishes, a suitable primer, primer surfacer or primer filler coating is applied over the multi-layer finish that usually is sanded thereby exposing one or more layers or is applied over a filler material that has been used to fill in surface imperfections. This primer, primer surfacer or primer filler, herein after, “primer”, has many requirements. It must have adhesion to the substrate and provide a surface to which the colored layer or top-coat will adhere. It must be readily sandable in a reasonably short period of time after application, for example, about three hours after application. It must provide the resulting multi-layer finish with good impact resistance, in particular, stone chip resistance.
For a primer to exhibit the above properties, the cured primer layer should have high flexibility at low temperatures combined with high hardness under low stress conditions. In general, soft primers that have a high degree of flexibility and chip resistance, have poor sandability due to their rubber like nature and hence, cannot be processed rapidly in an auto refinish operation, which results in lowered productivity. If a primer is used that can be readily sanded after a very short drying time, it generally is hard and has poor flexibility and chip resistance. That problem has been partially overcome by using activated base coats to improve the chip resistance of the resulting multi-layer finish. However, this further complicates the refinishing process with the use of another reactive coating that has a limited “pot life”.
There are several primer compositions that can be used. For example, Barsotti, et al. U.S. Pat. No. 6,221,494, teaches the use of a solvent based ambient temperature curable high solids urethane coating useful for refinishing automobile and truck bodies and parts. Harris, et al. U.S. Pat. No. 5,596,043 shows a powder coating composition containing urethane and acrylic resins useful for coating that have flexibility and stone chipping resistance and McNeil, et al. U.S. Pat. No. 6,210,758 also shows a coating composition of an acrylic polymer, a polyurethane and a crosslinking agent that has improved chip resistance.
In the refinishing of automobiles and trucks when a clear top coat finish is used, the clear top coat finish must dry to a tack free state after only a short period of time after application, for example, in about 30 minutes or less after application. This allows the vehicle to be moved from the spray area of a typical refinish establishment to another area where additional work can be performed on the vehicle, such as lightly sanding and/or buffing the clear top coat to improve the appearance and remove minor defects. At this stage, the clear top coat has to be cured to a sufficient level to allow for this buffing and/or sanding.
The novel composition of this invention can be readily formulated using conventional techniques to form finishes, in particular primer and clear coat finishes, that cure at low temperatures, preferably, ambient temperatures. This composition provides a primer composition having a combination of early hardness with good sandability. When such a primer is used in combination with a base-coat and clear-coat, a multi-layer finish is formed on an automobile or truck body or part that has good stone chip resistance. When used as a clear coat finish, the novel composition cures to a hard tack free finish in a relatively short period of time and forms a finish that has the required physical properties of exterior durability and weatherability. Also, the novel composition contains components that are derived from renewable resources.
It would be very desirable to have a liquid coating composition, in particular, a primer composition, or a clear coat composition that contains components that are derived from renewable resources and has the desired physical properties for primers and clear coats. The novel composition of this invention in the form of a clear coat or a primer meets these aforementioned requirements.